01 Civ- Ideal Exhaust Diameter?

Just wondering if anyone can point me in the right direction for sizing the pipe for my 01's exhaust. MAgnaflow sells a 2.25" system and they claim a "10% increase in power" But 2.25 seems like it's way to damn big....?

I am doing a complete exhaust system- hedman header, high flow cat, n1 style muffler. I will probably do a cold air intake as well but other than that, it's my DD and i just want to free up some ponies and maybe even some mpg